This invention relates to a system for reducing the concentration of elemental sulfur in physical scrubbing agents used for the removal from gases, e.g. coal gasification gas, of volatile sulfur compounds, such as hydrogen sulfide, sulfur dioxide, carbonyl sulfide and/or mercaptans.
In view of the relatively limited supply of gaseous and liquid fossil fuels, coal will be used to an increasing extent as the starting material for the production of hydrogen for the ammonia synthesis or for hydrogenation purposes, as well as for obtaining CO--H.sub.2 mixtures for either methanol or Fischer-Tropsch syntheses. For this purpose, the coal is partially oxidized with steam and oxygen or air, yielding a gas which contains in addition to hydrogen and carbon monoxide, considerable amounts of carbon dioxide and, as impurities, hydrogen sulfide, carbonyl sulfide, mercaptans and oxygen and/or sulfur dioxide.
Before the gases can be used for the various syntheses, they must first be subjected to a series of conversion and purifying steps. For example, when producing hydrogenation-grade hydrogen or ammonia syntheses gas, the carbon monoxide present in the gas is converted, i.e., reacted with steam, thus producing hydrogen and carbon dioxide. The carbon dioxide is then separated from the gas and often purified to the extent it can then be used for further syntheses, for example for urea synthesis. Conversely, if it is desired to produce methanol synthesis gas or Fischer-Tropsch synthesis gas, i.e., mixtures of carbon monoxide and hydrogen, then only a partial conversion of any excess carbon monoxide is conducted, as required.
Irrespective of the synthesis involved, in all cases the sulfur compounds must be removed from the gas before the actual synthesis, since such sulfur compounds act, inter alia, as catalyst poisons. To remove sulfur compounds from gases, along with high concentrations of carbon dioxide, the so-called physical scrubbing methods have proven to be effective for many years. During such scrubbing processes, no chemical reaction takes place between the components to be removed and the scrubbing agent; rather, merely a solution occurs, the extent of which depends essentially on the solubilities of the respective components in the scrubbing agent, which in turn are dependent on the pressure and temperature employed. The regeneration of such scrubbing agents takes place by expansion and/or heating and/or by stripping with a gas having very low partial pressures of the components to be removed.
Highly suitable physical scrubbing agents for such components, e.g. H.sub.2 S and CO.sub.2, are polar organic scrubbing agents, and especially lower monohydric alcohols, such as methanol, and also ketones, such as acetone, and polyhydric alcohols, such as ethylene glycol.
Since the solubility of the components to be scrubbed generally increases with increasing pressure and with decreasing temperature, such scrubbing processes, e.g., methanol scrubbing steps, are conducted under pressures of up to 150 atmospheres absolute and at temperatures down to -70.degree. C., usually in the range of under 10 to 80 atmospheres and at 0.degree. C. to -60.degree. C.
In a conventional process of this type (DAS (German Published Application) No. 1,567,690), a coal gasification gas is, after quenching and cooling, first freed of H.sub.2 S and COS in a first scrubbing stage with methanol and then warmed; the CO in the gas is converted to CO.sub.2 ; the resultant gaseous mixture is cooled and then freed of CO.sub.2 in a second scrubbing stage, likewise with the aid of methanol. The remaining gas, containing residual impurities of carbon monoxide, methane and argon, is freed of these components in a scrubbing process with liquid nitrogen and fed to the ammonia synthesis in the form of a nitrogen-hydrogen mixture.
Difficulties are encountered in this process if the coal gasification gas, depending on the type of raw material, on the type of gasification method and on the pre-purification steps employed, contains O.sub.2 and/or SO.sub.2 in addition to hydrogen sulfide or if the H.sub.2 S containing scrubbing solution is brought into contact with an O.sub.2 and/or SO.sub.2 containing gas, e.g. a stripping gas. As has been observed, elemental sulfur soon precipitates in the scrubbing agent even with the presence of concentrations in the order of a few p.p.m. of the O.sub.2 and SO.sub.2 in the gases concerned. Such elemental sulfur results in the clogging of the apparatus and pipelines, thus causing havoc in the process.
These difficulties might as well be encountered in H.sub.2 S removal processes applied to other gases than coal gasification gas, e.g. all gases produced by partial oxidation of hydrocarbons, natural gas, hydrodesulfuration off-gases and reformer purge gas, if oxygen and/or sulfur dioxide is contained or formed in or likewise brought into these gases or if oxygen and/or sulfur dioxide is brought into the scrubbing agent.
The formation of elemental sulfur can probably be explained by the fact that a type of Claus process takes place among H.sub.2 S and SO.sub.2, and/or that H.sub.2 S is oxidized by O.sub.2 in the scrubbing agent, especially if it is warmed for regeneration purposes, leading to the formation of element sulfur.